How do I do A/B split testing with an Automated Workflow or Campaign?

I'm setting up something somewhat like an emailed course and using the Automated Workflow.

I'd like A/B split test some subject:'s.

I can't see where to do that.

I have gotten to this screen:

And the next choice is Start Workflow but I don't want to actually startit yet, I want to define A/B tests first.

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